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BACKGROUND OF THE INVENTION
This invention relates to a signal transmission device and a signal
reception device communicable with the signal transmission device and to a
communication method of carrying out communication between the signal
transmission device and the signal reception device.
In a conventional communication method of the type described, a voice
signal is often subjected to adaptive delta modulation (often abbreviated
to ADM) to be transmitted from a signal transmission device to a signal
reception device in the form of a sequence of digital voice signals which
may be called an ADM digital signal sequence. It is known in the art that
a sequence of information signals can be transmitted in combination with
such a digital voice signal sequence.
As a rule, the information signal sequence is transmitted in the form of a
non-return-to-zero signal from the signal transmission device to the
signal reception device. In this event, the information signal sequence
must be distinguished from the digital voice signal sequence in the signal
reception device. Otherwise, the information signal sequence might
objectionably audibly be reproduced together with the digital voice signal
sequence in the signal reception device. Such an audible reproduction of
the information signal sequence becomes noisy for listeners of the voice
signal and might sound like a beep or a buzz. As a result, the listeners
may become uncomfortable or uneasy.
Exact distinction of the information signal sequence from the digital voice
signal sequence tends to bring about an objectionable increase of an error
detection rate in the signal reception device because the digital voice
signal sequence is often wrongly detected as the information signal
sequence.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a communication method which
is capable of avoiding an audible reproduction of a sequence of
information signals without an increase of an error detection rate.
It is another object of this invention to provide a signal transmission
device which can be used for a communication method of the type described
and which can produce a sequence of output signals useful for suppression
of the audible reproduction of information signal sequence.
It is still another object of this invention to provide a signal reception
device which is communicable with a signal transmission device of the type
described and which can suppress the audible reproduction of information
signal sequence.
A signal transmission device to which this invention is applicable is for
use in transmitting a sequence of output digital signals. The signal
transmission device comprises adaptively converting means for adaptively
converting a voice signal into a sequence of digital voice signals and
information signal producing means for producing a sequence of information
signals. According to an aspect of this invention, the signal transmission
device comprises phase modulation means coupled to the information signal
producing means for phase-modulating the information signal sequence into
a binary Manchester code sequence and combining means coupled to the
adaptively converting means and the phase modulation means for combining
the digital voice signal sequence with the binary Manchester code sequence
into the output digital sequence.
According to another aspect of this invention, there is provided a
communication method of transmitting a sequence of output digital signals
from a signal transmission device to a signal reception device. The signal
transmission device comprises adaptively converting means for adaptively
converting a voice signal into a sequence of digital voice signals and
information signal producing means for producing a sequence of information
signals. According to this invention, the method comprises the steps of
phase-modulating, in the signal transmission device, the information
signal sequence into a binary Manchester code sequence, combining, in the
signal transmission device, the digital voice signal sequence with the
binary Manchester code sequence into the output digital sequence,
reproducing, in the signal reception device, the digital voice signal
sequence into a primary reproduction of digital voice signal sequence in
response to output digital signal sequence, and phase-demodulating, in the
signal reception device, the binary Manchester code sequence into an
additional reproduction of the information signal sequence in response to
the output digital signal sequence.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of a conventional communication system which can
carry out transmission of a sequence of digital signals subjected to
adaptive delta modulation;
FIG. 2 shows a signal format for use in describing a sequence of output
signals transmitted in
FIG. 3 shows a waveform for use in describing the output signal sequence
illustrated in FIG. 2;
FIG. 4, depicted below FIG. 2, shows another signal format of a sequence of
output signals for use in another conventional communication system and
this invention;
FIG. 5 is a block diagram of a communication system according to a first
embodiment of this invention;
FIG. 6, depicted below FIG. 3, shows a diagram for use in describing a
waveform used in the communication system illustrated in FIG. 5; and
FIG. 7 is a block diagram of a communication system according to a second
embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, a conventional communication network will be
described for a better understanding of this invention. The illustrated
network comprises a transmitter section 11 and a receiver section 12 which
is communicable with the transmitter section 11 through a radio channel.
The transmitter section 11 and the receiver section 12 may be called a
signal transmission device and a signal reception device, respectively.
The transmitter section 11 comprises a microphone 13 for transforming a
speaker's voice into an electric voice signal which will be simply
referred to as a voice signal. Responsive to the voice signal, an adaptive
delta encoder 14 carries out adaptive delta modulation of the voice signal
to produce a sequence of digital voice signals. As a result, the digital
voice signal sequence is subjected to adaptive delta modulation.
As known in the art, it is possible to transmit a sequence of additional or
subsidiary information signals in a quiescent or absent duration which
inevitably occurs in such a digital signal sequence. Such a quiescent
duration results from a pause of the speaker's voice which defines a
voiceless state. The subsidiary information signal sequence may be a
status signal, a calling signal, an identification signal, and the like
and will be called a sequence of information signals INF. To this end, the
digital voice signal sequence is delivered to both of a combining circuit
15 and a voiceless state detector 16. When a voiceless state as mentioned
above is detected by the voiceless state detector 16, a signal generator
17 is energized to produce a sequence of information signals as mentioned
above in a manner to be presently described. In the example being
illustrated, each of such information signals follows a start signal which
is representative of start or beginning of each information signal and
which will be depicted at ST hereinafter. As illustrated in FIG. 2, a
specific one of the information signal is depicted at INF and interposed
within the quiescent duration between two adjacent ones of the digital
voice signals. Thus, each information signal may be produced after speech
or communication.
The signal generator 17 is coupled to an operator control panel 18 having a
plurality of keys arranged thereon. With the operator control panel 18, it
is possible to supply the signal generator 17 with various kinds of
control signals. Such supply of the control signals is possible by
manipulating the keys on the operator control panel 18. The signal
generator 17 is also coupled through the combining circuit 15 to a
transmitter 19 enabled by a transmission switch 21 operated by an
operator. In the illustrated example, the transmission switch 21 is closed
and opened when speech or communication is started and completed,
respectively.
Let a specific one of the control signals be produced from the operator
control panel 18 and be made to correspond to the specific or selected
information signal. In this event, the signal generator 17 produces the
start signal ST followed by the selected information signal INF each time
when the signal generator 17 is energized by the voiceless state detector
16. The start signal ST and the selected information signal INF are sent
from the signal generator 17 to the combining circuit 15 and are combined
with the digital voice signal sequence into a sequence of output digital
signals.
More particularly, the digital voice signal sequence is sent to the
transmitter 19 with the pauses left between adjacent ones of the voice
signals as long as the transmission switch 21 is closed. On the other
hand, the selected information signal INF is transmitted from the
transmitter 19 within each pause. Moreover, the selected information
signal INF may be transmitted when the transmission switch 21 is opened
after completion of communication or speech. At any rate, the output
digital signal sequence is transmitted to the receiver section 12 through
the radio channel in the form of a radio wave.
Temporarily referring to FIG. 2, a signal format of the output digital
sequence has first, second, and third signal areas 26, 27, and 28 assigned
to the digital voice signal, the start signal ST, and the information
signal INF, respectively. As shown in FIG. 2, the start signal ST is
produced when the transmission switch 21 is opened after completion of
communication or speech.
Referring to FIG. 3 together with FIG. 1, the signal generator 17 (FIG. 1)
produces the start signal ST and the information signal INF after
production of the digital voice signal. The start signal ST has a
predetermined start pattern of eight bits at a predetermined period T. The
information signal INF follows the start signal and has a bit period
identical with the predetermined period T. As readily understood, each of
the start and the information signals ST and INF is formed by a
non-return-to-zero (NRZ) signal.
Referring back to FIG. 1, the output signal sequence is received as a
sequence of reception signals by the receiver section 12. In the receiver
section 12, the reception signal sequence is supplied to a receiver 31
coupled to a delay circuit 33, a start signal detector 34, and a squelch
circuit 35. The reception signal sequence is successively sent through the
receiver 31 and the delay circuit 33 to a decoder 36 which carries out
demodulation or decoding of the digital voice signal sequence carried by
the reception signal sequence. Inasmuch as the digital voice signal
sequence is subjected to the adaptive delta modulation as mentioned
before, the decoder 36 may be called an ADM decoder and decodes the
digital voice signal sequence into a reproduction of the digital signal
sequence which may be referred to as a primary reproduction.
On the other hand, the start signal detector 34 monitors the reception
signal sequence to detect the start signal ST of the start pattern as
shown in FIG. 3 and to produce a detection signal DT indicative of
detection of the start signal ST. The detection signal DT is sent to an
information detector 38 along with the reception signal sequence given
through the receiver 31. Responsive to the detection signal DT, the
information detector 38 is put into a reception state of the information
signal sequence following the start signal ST. At any rate, a reproduction
of the information signal sequence is delivered from the information
detector 38 to a display device 39 to be displayed thereon and will be
called an additional reproduction.
The detection signal DT is also sent to a timer 41 for timing or measuring
a preselected interval of time. More specifically, the timer 41 keeps a
logic "0" level during the preselected interval of time in response to the
detection signal DT. Accordingly, the receiver section 41 is put into a
reception state of the information signal sequence INF during the
preselected interval as will become clear as the description proceeds.
The squelch circuit 35 judges presence or absence of the reception signal
sequence to produce, as a squelch signal, a logic "1" level signal and a
logic "0" level signal, respectively. Such judgement of the reception
signal sequence is possible by monitoring a field strength. The squelch
signal is delivered to an AND gate 42 together with a timer output signal
supplied from the timer 41.
Consequently, the AND gate 42 produces the logic "1" level signal during
presence of the reception signal sequence until the start signal ST is
detected by the start signal detector 34. In other words, the logic "0"
level signal is produced from the AND gate 42 during the reception state
of the information signal INF defined by the timer 41. A switch 44 is
closed in response to the logic "1" level signal sent from the AND gate 42
while the switch 44 is opened in response to the logic "0" level signal.
This shows that the switch 44 is closed during the reception of the
digital voice signal sequence and is opened during the reception of the
information signal sequence. Therefore, the reproduction of the digital
voice signal alone is sent through the switch 44 to a loudspeaker 45 and
transformed into a sound with the reproduction of the information signal
sequence suppressed by the switch 44.
In order to favorably carry out the above-mentioned operation, the start
signal ST must exactly be detected by the start signal detector 34.
However, detection of the start signal ST is made on the assumption that
occurrence of an error is acceptable to some extent on the detection of
the start signal ST because an error rate is comparatively high on a
transmission line for transmitting such an output signal sequence carrying
a combination of the digital voice signal sequence and the information
signal sequence. Therefore, the information signal sequence is not exactly
distinguished from the digital voice signal sequence at the receiver
section 12.
Taking the above into consideration, let a permissible error be restricted
to a low value in the receiver section 12. In this event, a detection rate
of the start signal ST becomes undesirably low. As a result, the switch 44
is unfavorably kept in a closed state even during the reception of the
information signal sequence. Under the circumstances, the information
signal sequence is undesirably reproduced by the loudspeaker 45 as a noisy
sound.
On the other hand, let the permissible error be set at a high value in the
receiver section 12 to increase the detection rate of the start signal ST.
In this case, a false start signal may be wrongly detected from the
digital voice signal sequence with a high probability. Consequently, the
digital voice signal sequence might be undesirably interrupted by
detection of such a false start signal.
At any rate, the switch 44 might often be wrongly opened during the
reception of the digital voice signal sequence or closed during the
reception of the information signal sequence. Such wrong detection of
either the digital signal sequence or the information signal sequence
brings about either undesirable interruption of the digital voice signal
sequence or objectionable occurrence of a noisy sound.
Referring to FIG. 4, another conventional communication system carries out
communication between a transmitter section and a receiver section by the
use of another sequence of output signals that has a signal format as
exemplified in FIG. 4. As shown in FIG. 4, the information signal INF is
preceded by the start signal ST like in FIG. 2 and transmitted after
completion of transmission of the digital voice signal sequence. In order
to produce the illustrated output signal sequence, the transmitter section
may detect only the completion of the transmission by monitoring a
transmission switch as depicted at 21 in FIG. 1. Therefore, the
transmission switch may be coupled to the signal generator 17 (FIG. 1) in
addition to the transmitter 19. With this structure also, similar problems
inevitably occur in the receiver section like in FIG. 1 even when such an
output signal sequence is used, because the start signal ST might wrongly
be detected by the receiver section.
Referring to FIG. 5, a communication system according to a first embodiment
of this invention comprises a transmitter section 11a and a receiver
section 12a both of which comprise similar parts designated by like
reference numerals. As shown in FIG. 5, the illustrated transmitter
section 11a further comprises a phase modulator 51 coupled to the
voiceless state detector 16, the signal detector 17, and the combining
circuit 15. More specifically, the phase modulator 51 is enabled when the
voiceless state is detected by the voiceless state detector 16. To this
end, the voiceless state detector 16 supplies the phase modulator 51 with
a voiceless state signal VL representative of detection of the voiceless
state.
When the phase modulator 51 is enabled or energized by the voiceless state
detector 16, the signal generator 17 is also enabled by the voiceless
state detector 16 in the manner described in conjunction with FIG. 1. In
this situation, the signal generator 17 at first supplies the phase
modulator 51 with the start signal ST and the information signal sequence
INF after the start signal ST in the manner illustrated with reference to
FIG. 1. It is to be noted here that each of the start signal ST and the
information signal sequence INF is formed by a non-return-to-zero signal,
as already described in conjunction with FIG. 3.
In the example being illustrated, the phase modulator 51 is for converting
the information signal sequence INF into a binary Manchester code
sequence.
Referring to FIG. 6 in addition to FIGS. 3 and 5, the start signal ST and
the information signal sequence INF are converted into a modulated start
signal STm and a modulated information signal sequence INFm both of which
are interposed within a quiescent duration between two adjacent ones of
the digital voice signals, like in FIG. 3. As readily understood from FIG.
6, each of the modulated start signal STm and the modulated information
signal sequence INFm is a binary Manchester code sequence. More
particularly, each bit of the start signal ST and the information signal
sequence INF has the predetermined signal period T and is made to
correspond to each Manchester code which lasts for 2 T. In each Manchester
code, the logic "1" level of the start signal ST and the information
signal sequence INF is converted into a first phase changed from a low
level to a high level while the logic "0" level is converted into a second
phase changed from the high level to the low level. At any rate, the first
and the second phases are different from each other by .pi. radians. More
specifically, the first phase exhibits a waveform built up at a central
time instant while the second phase another waveform built down at a
central time instant. Use of such a Manchester code makes it possible to
minimize a reduction of a transmission efficiency.
A single bit of the start signal ST and the information signal sequence INF
may be made to correspond to another phase modulated code lasting for a
signal duration of 2 nT where n is a natural number, although the
transmission efficiency is reduced in comparison with use of the
Manchester code sequence. In this event, the phase modulator 51 produces,
for a single bit of the start signal ST and the information signal
sequence INF, either a first iterative pattern of the first and the second
phases or a second iterative pattern of the second and the first phases.
In any event, such a phase modulated signal makes it possible to avoid an
audible reproduction of the information signal sequence INF, as will
become clear later. For the time being, it may be understood that the
non-return-to-zero signal has a signal component distribution concentrated
in a low frequency region while the phase-modulated signal has another
signal component distribution different from the non-return-to-zero
signal. Specifically, the phase-modulated signal has a reduced amount of a
low frequency component and a large amount of a high frequency component
in a high frequency region. The low and the high frequency regions may be
called a first and a second frequency regions, respectively.
In FIG. 5, the phase-modulated signal is delivered from the phase modulator
51 to the combining circuit 15 responsive to the digital voice signal
sequence which is subjected to the adaptive delta modulation in the
adaptive delta encoder 14. The adaptive delta encoder 14 may be, for
example, MC3417, MC3517, manufactured and sold by Motorola Inc. The
phase-modulated signal is interposed within the quiescent time of the
digital voice signal sequence. As a result, the phase-modulated signal is
combined with the digital voice signal sequence by the combining circuit
15 to be transmitted through the transmitter 19 to the receiver section
12a as a sequence of output signals. With this structure, the digital
voice signal sequence is produced as the output signal sequence during
closure of the transmission switch 21 while the phase-modulated signal is
produced as the output signal sequence during the quiescent time or in the
absence of the digital voice signal sequence and during an off state of
the transmission switch 21.
The output signal sequence is received by the receiver section 12a as a
sequence of reception signals. The illustrated receiver section 12a is
similar in structure to that illustrated in FIG. 1 except that a phase
demodulator 52 is included in the receiver section 12a. The phase
demodulator 52 is coupled to the squelch circuit 35, the receiver 31, and
the information detector 38. The squelch circuit 38 delivers the logic "1"
level signal to the AND gate 42 during reception of the reception signal
sequence as mentioned in conjunction with FIG. 1. The illustrated squelch
circuit 38 also delivers an enable signal to the phase demodulator 52
during reception of the reception signal sequence. Thus, the enable signal
and the logic "1" level signal are sent to the phase demodulator 52 and
the AND gate 42 while a squelch operation is not carried out in the
squelch circuit 35. Responsive to the enable signal, the phase demodulator
52 sends a sequence of demodulated signals to the information detector 38.
Such a demodulated signal sequence does not make sense during reception of
the digital voice signal sequence carried by the reception signal sequence
and is significant only on reception of the information signal sequence
INF.
In order to derive only the information signal sequence INF from the
reception signal sequence, the demodulated signal sequence is sent to the
information detector 38 which is supplied with the detection signal DT
from the start signal detector 34 like in FIG. 1. The information detector
38 is energized in response to the detection signal DT to detect the
demodulated signal sequence and to produce a sequence of detected signals
which may be a reproduction of the start signal ST and the information
signal sequence INF. The detected signal sequence is displayed on the
display device 39 in the manner described with reference to FIG. 1. Even
when the modulated start signal STm and the modulated information signal
sequence INFm is used to transmit the start signal ST and the information
signal sequence INF, the start signal detector 34 might wrongly detect
such a modulated start signal STm. As a result, the switch 44 might
wrongly be closed or opened, as mentioned in FIG. 1. However, wrong
operation of the start signal detector 34 can be substantially compensated
in a manner to be described below.
Like in FIG. 1, the reception signal sequence is sent through the receiver
31, the delay circuit 33, and the adaptive delta modulation decoder 36 to
the switch 44 which might wrongly be operated. It is mentioned that an
adaptive delta decoder can generally produce an analog signal by
integrating an input digital signal sequence. On the other hand, it is to
be recollected that the phase-modulated signal, such as the modulated
start signal STm and the modulated information signal sequence INFm,
comprises a reduced amount of the low frequency component. The amount of
the low frequency component decreases with an increase of the factor n
determined in relation to the signal period of the phase-modulated signal.
Therefore, an amplitude or energy of the phase-modulated signal becomes
small even when such a phase-modulated signal is wrongly sent from the
decoder 36 through the switch 44 to the loudspeaker 45. This shows that
the phase-modulated signal is not audible for listeners even if it is
reproduced together with the digital voice signal sequence. Anyway, a
combination of the delay circuit 33, the decoder 36, the switch 36, and
the loudspeaker 45 may serve to reproduce the digital voice signal
sequence and may be referred to as a reproducing circuit for producing a
reproduction of the digital voice signal sequence.
With this structure, it is possible to lower an error detection rate of the
start signal ST. In other words, no noisy sound is audible even on false
detection of the start signal ST. In this manner, intermittent
interruption of a reproduced voice can practically becomes negligible in
such a communication system.
Referring to FIG. 7, a communication system according to a second
embodiment of this invention comprises similar parts designated by like
reference numerals. As is readily understood by FIG. 7 with FIG. 5, the
illustrated transmitter section depicted at 11b does not comprise a
voiceless state detector 16 as shown in FIG. 5. In addition, the
transmission switch 21 is connected not only to the transmitter 19 but
also to the signal generator 17. It is assumed that the output signal
sequence is transmitted from the transmitter section 11b to a receiver
section (depicted at 12b) only during closure of the transmission switch
21.
Like in FIG. 4, it is further assumed that the start signal ST and the
information signal sequence INF are produced by the signal generator 17
after completion of transmission switch 21 kept closed. Such production of
the start signal ST and the information signal sequence INF is possible
after transmission of the digital signal sequence because the transmission
switch 21 is not instantly put into an off state after completion of
communication or speech.
Likewise, a combination of the start signal ST and the information signal
sequence INF may be produced by the signal generator 17 before
transmission of the digital voice signal sequence, as illustrated at a
broken line in FIG. 5 because communication or speech is not started
immediately after closure of the transmission switch 21.
At any rate, the start signal ST and the information signal sequence INF
are sent from the signal generator 17 to the phase modulator 51 at the end
or the beginning of speech or communication in a manner similar to that
described in conjunction with FIG. 5. Consequently, the start signal ST
and the information signal sequence INF are subjected to phase modulation
in the manner mentioned with reference to FIGS. 5 and 6 and are modulated
into a phase modulated signal. Such a phase modulated signal may be either
a binary Manchester code signal or the like, as described in relation to
FIG. 5 and therefore has a reduced amount of a low frequency component
falling within a low frequency region or the first frequency region. As
long as the phase modulated signal, such as the binary Manchester code
signal or the like, is used in the communication system, a reproduction of
the modulated information signal sequence INFm accompanies no noisy sound
for listeners, as is the case with FIG. 5.
The output signal sequence is received as a sequence of reception signals
by the receiver section 12b. The illustrated receiver section 12b is
similar in structure and operation to that mentioned in FIG. 5 except that
a phase demodulator 52 is not connected to the squelch circuit 35. This is
because no reception signal sequence is received after the transmission
switch 21 is closed in the transmitter section 11b.
Thus, the reception signal sequence is delivered from the receiver 31 to
the adaptive delta modulation decoder 35 through the delay circuit 33 on
one hand and to both of the start signal detector 34 and the phase
demodulator 52 on the other hand. When the detection signal DT is supplied
from the start signal detector 34 to the information detector 38 as a
result of detection of the modulated start signal STm, the modulated
information signal sequence INFm is delivered from the phase demodulator
42 to the information detector 38 as a sequence of demodulated signals,
like in FIG. 5. The information detector 38 detects the demodulated signal
sequence and supplies the same to the display device 39 in the manner
described before.
In the example being illustrated, the modulated start signal STm might
wrongly be detected by the start signal detector 34. This gives rise to
wrong closure of the switch 44 because the detection error of the
modulated start signal STm is allowable to some extent in the illustrated
communication system. Under the circumstances, the phase modulated signal,
especially, the modulated information signal sequence INFm passes through
the switch 44 and is sent to the loudspeaker 45 to be reproduced. However,
such a reproduction of the modulated information signal sequence INFm is
not audible, as already described before. Accordingly, it is possible to
avoid intermittent interruption of speech or communication if the error
detection rate is selected at a low value like in FIG. 5.
Thus, transmission is carried out by the use of a phase modulated signal
which scarcely comprises a low frequency component and which is supplied
to a decoder, such as an adaptive delta modulation decoder, for deriving a
low frequency component by integration. As a result, the low frequency
component of the phase modulated signal is negligibly small and is not
noisy for listeners. When there is a receiver section wherein reception of
any information signals is unnecessary, an error detection rate of the
modulated start signal STm may not be high enough in such a receiver
section. Therefore, no intermittent interruption of speech or voice
practically takes place in the receiver section.
While this invention has thus far been described in conjunction with a few
embodiments thereof, it will readily be possible for those skilled in the
art to put this invention into practice in various other manners. For
example, the timer 41 and the delay circuit 33 illustrated in FIGS. 5 and
7 may be omitted from each receiver section when no problem occurs about
signal quality in the voiceless state or when the number n determined for
the phase modulated signal is very large.
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